1. Field of the Invention
The present invention relates in general to automatic washing machines using low frequency vibration to wash clothes and, more particularly, to an improved structure in washing tubs of such washing machines for causing smooth circulation of water current and giving frictional force to clothes circulated in the water current.
2. Description of the Prior Art
In order to wash clothes in a conventional automatic washing machine using low frequency vibration, a low frequency oscillating disc placed in a washing tub generates a specified low frequency vibration which causes resonance phenomena in a multi-phase washing medium in the washing tub. Here, the multi-phase washing medium is made of water, detergent and air. The level of low frequency is specified in accordance with the shape of the washing tub, the shape of the oscillating disc and the mixing ratio of the multi-phase washing medium.
In the above washing machine, the desired mechanical washing effect is achieved by the micro air bubbles generated by cavitation of the oscillating disc or nonlinear vibration. Both the cavitation and the nonlinear vibration are generated in the multi-phase medium at the same time as the generation of the resonance phenomena. The mechanical washing effect caused by the cavitation or the nonlinear vibration cooperates with a chemical washing effect caused by the detergent in the washing medium, thus to achieve the desired washing effect.
With reference to FIG. 1, there is shown a typical automatic washing machine using low frequency vibration. The washing machine generally comprises a washing tub 1a receiving the multi-phase washing medium therein. A low frequency oscillating disc 5 causing the resonance phenomena in the washing medium is placed in the washing tub 1a and coupled to an actuator 3 through a shaft 4. The shaft 4 transmits the drive force of the actuator 3 to the disc 5 so as to drive this disc 5. The actuator 3, which is mounted on the outer bottom of the washing tub 1a, is applied with an amplitude signal and a frequency signal from a signal oscillator 6. A signal amplifier 7 is placed on a signal line between the actuator 3 and the signal oscillator 6 for amplifying and varying the signals generated by the signal oscillator 6.
There is provided a gap between the inner bottom of the washing tub 1a and the disc 5 for allowing the disc 5 to vertically vibrate. A lid 8 is provided in the opening of the washing tub 1a for hermetically covering the opening.
In operation of the above washing machine, the low frequency oscillating disc 5 is driven by the actuator 3 in response to signals applied from the signal oscillator 6 to the actuator 3 through the signal amplifier 7. This disc 5 thus vibrates in order to cause the resonance phenomena in the multi-phase washing medium in the washing tub 1a. Here, the desired mechanical washing effect is achieved by the micro air bubbles generated by cavitation of the oscillating disc 5 or nonlinear vibration, which cavitation and nonlinear vibration are generated in the multi-phase medium at the same time as the generation of the resonance phenomena. The above mechanical washing effect cooperates with the chemical washing effect caused by the detergent in the washing medium, thus to achieve the desired washing effect.
In washing clothes using the low frequency vibration, the signal oscillator 6 makes the disc 5 vibrate in an oscillation frequency band of 20-250 Hz, an amplitude band of 2-25 mm and a rotational amplitude band of 2.degree.-10.degree.. Here, the signal amplifier 7 amplifies current signals or voltage signals which are to be applied from the signal oscillator 6 to the actuator 3.
However, it has been noted that the above washing machine has the following problems.
Since the side wall of the cylindrical washing tub 1a meets with the bottom of the washing tub 1a at right angles, the washing tub 1a can not smoothly guide the heart type or scroll type current of washing medium. This disturbs the clothes from smooth circulation in the current and deteriorates the washing effect.
In addition, the washing tub 1a of the low frequency vibration washing machine is simply flat on the inner surface of its side wall in contrast with the washing tub 1b of a typical pulsator type washing machine of FIGS. 2A and 2B. With the flat surface, the washing tub 1a gives produces no frictional force to the clothes circulated in the washing medium. Frictional force is known to cause a so-called "washboard effect" that doubles the washing effect. In this regard, the washing tub 1a does not achieve the desired washing effect.
FIGS. 2A and 2B show the washing tub 1b of a typical pulsator type washing machine. In washing clothes washing in the pulsator type washing machine, the clothes are circulated along with the washing medium current in the washing tub 1b as a result the rotation of a pulsator (not shown). Here, the inner surface of the side wall of the washing tub 1b is uneven so as to form a plurality of friction prominences 2a of the vertical stripe type. With the uneven structure, the washing tub 1b produces a frictional force, which causes the washboard effect and doubles the washing effect, to the clothes circulated in the washing medium. The frictional force caused by the washing tub 1b having the uneven inner side surface makes the clothes cleaned as if they were manually washed using a washboard.